Hatch covers and ventilating system

ABSTRACT

A vessel of the OBO type in which the holds are serviced by a pair of transversely disposed hatches and a central longitudinal girder forms a ventilating trunk extending lengthwise of the vessel and a support for the adjacent hatch coamings of the transversely disposed hatches. The hatch covers are raised and lowered by jacks and one of the pair of hatch covers of the transversely disposed hatches is movable transversely of the vessel to underneath the other of the pair of hatch covers, and both of the pairs of hatch covers are movable piggyback transversely of the vessel.

HATCH COVERS AND VENTILATING SYSTEM 1 Oct. 17,1972

FOREIGN PATENTS OR APPLICATIONS 1,196,791 7/1970 Great Britain ..114/202 [72] Inventor: Norman V. Laskey, 3641 Aylmer Street, MontrealQuebec, Canada Primary Examiner-Andrew Farrell [22] i Jam 5, 7Attorney-Jerry B. Cesak A vessel of the OBO type in which the-holds areser- [30] Forms Apphcauon Pnomy Data viced by a pair of transverselydisposed hatches and a May 19, 1970 Great Britain ..24,09l/70 centrallongitudinal girder forms a ventilating trunk extending lengthwise ofthe vessel and a support for [52] US. Cl. ..ll4/202 the adjacent hatchcoamings of the transversely [ITI- Cl. disposed hatches The batch coversare raised and 1 Field of Search 14/202; 49/ 8 lowered by jacks and oneof the pair of hatch covers 2 of the transversely disposed hatches ismovable transversely of the vessel to underneath the other of the [56]Reterences C'ted pair of hatch covers, and both of the pairs of hatchUNITED STATES PATENTS covers are movable piggyback transversely of thevessel. 1,768,701 7/1930 MacGregor ..l14/202 1,849,801 3/1932 MacGregor..1 14/202 12 Claims, 11 Drawing Figures [13! 1 1 e r i e i L 56 1 A 4Q1 35? 31; 57 -23 rm 5; 45-) p t I 29 L\ g 25 25 g 24 45 3' I PATENTEnucI11 m2 3. 698.347

sum 3 0F 5 I F/Q INVENTOR N. v. LASKEY PATENTEBncnnszz 3.698 347 Vsnmuur s INVENTOR N. V. LASKEY PATENT AGENTS FATE "TED 17 I973 3.6 98347 sum 5 or 5 INVENTOR N. V. LASKEY PATENT AYJENTS 1 HATCH COVERS ANDVENTILATING SYSTEM This invention relates to vessels of the 080 or Oil-Bulk-Ore type and particularly to the Hatch Covers and Ventilationsystems employed in such vessels.

Over the past years, single deck bulk carriers of the self-trimmingtype, comprising upper-wing or saddle ballast tanks have become popularand shipowners continue to order them in large numbers. The reason forthis is not far to seek, because it is predicted that seaborne tradewill increase at the rate of 6.5 percent per year over the next 15 yearswhile the demand for shipping will be in the region of 7.25 percent.Also, dry bulk cargoes to betransported in ships will increase at therate of about 9 percent each year. i

It is of interest to note that the original design intent of the singledeck bulk carrier was to provide a vessel suitable for handling alltypes of free flowing bulk cargoes such as grain, coal, phosphate rock,potash, bauxite, alumina as well as manganese and iron ores in thenatural, concentrated or pelletized forms. However, these vessels arebeing used with success in transporting automobiles, forest productsincluding logs, scrap and finished steel products.

In the design of bulk carriers, the continued use of a single hatch witha width equal to 50 percent of the beam of the vessel is in withupper-wing or saddle ballast tanks has, today, outlived its usefulness.In the past, this was an essential feature of a single deck bulk carrierin order to provide self-trimming features for the carriage of grain inbulk and to eliminate the need for center-line shifting boards. Thisarrangement is also alleged to permit an adequate quantity of ballast tobe carried during a ballasted voyage. I-Iowever,'with such anarrangement, the underdeck cubic in-way of the holds available for thecarriage of a bulk cargo is reduced by the cubic invested in theupper-wing or saddle ballast tanks.

On many bulk carriers designed with the machinery at the after end, thehogging stress in the ballasted condition, using upper-wing or saddletanks, will be at an inadmissible level. Sea-keeping performance in theballasted condition will be greatly improved if water ballast is carriedin a large number of the cargo holds instead of in upper-wing or saddletanks.

A twin-hatch open ship of the type herein shown can be certified for thecarriage of grain in bulk without shifting boards in full compliancewith the requirements of SOLAS 1960. In view of this, provision can bemade to carry water ballast in all the cargo holds if need be so as toensure better sea-keeping performance in the ballasted condition.

The provision of twin hatches per hold, oriented transversely across thevessel, applies equally as well in vessels of the 080 type whereupper-wing or saddle tanks are eliminated. In fact, with twin hatches,the free surface correction to be deducted from the GM owing to thepresence of a liquid within the square of the hatch will be less with anopen ship arrangement than with a conventional bulk carrier arrangementhaving upper-wing or saddle tanks.

It is, therefore, an object of the present invention to provide a vesselof the open ship concept, serviced by pairs of hatches on the main deck.

It is a further object of the invention to provide a pair of hatchcovers for each hold opening, with one of the pair of hatch covers beingelevated to a height to enable the other of the pair of hatch covers tobe elevated a short distance to clear its sealing joint and betransported sideways under the first mentioned elevated hatch cover.

A further object of the invention is to provide means whereby adjacentpairs of hatch covers can be transferred piggy back transversely of thevessel for ease of working both sides of the hold.

A further object of the invention is to provide a centerlinelongitudinal structure which is used to accommodate a part of the cargohatch transportation mechanism, to provide a ventilation duct and tocontrol ullage below deck.

A further object of the invention is to provide a positive gas seal foreach of the pairs of hatch covers.

A still further object of the invention is to provide a system forventilating the cargo holds incorporated in the hatch coamings of theside-by-side hatch openings.

These and other objects of the invention will be apparent from thefollowing detailed specification and the accompanying drawings in whichFIG. 1 is a plan view at deck level of a vessel incorporating thepresent invention.

FIG. 2 is a plan view under deck level showing a proposed holdarrangement.

FIG.3 is a transverse section taken on the line 3-3 of FIG. 1.

FIG. 4 is a partial transverse section of the vessel showing a pair ofside-by-side hatch covers in the locked-shut position on the hatchcoamings.

FIG. 5 is a view similar to FIG. 4, but showing one of the pair of hatchcovers elevated above the level of the other of the pair of hatchcovers.

FIG. 6 is a view similar to FIG. 5, but showing the other of the pair ofbatch covers slid transversely under the first elevated hatch cover,leaving one hatch completely open.

FIG. 7 is an enlarged vertical sectional view taken on the line A-A ofFIG. 4 showing the lifting rail and hatch cover in the raised position.

FIG. 8 is an enlarged vertical, sectional view on the line B-B of FIG. 5showing the lifting rail in the lowered position and the hatch cover inthe locked-shut position.

FIG. 9 is an enlarged view of the area at the ships centerline as seenin FIG. 5 with one hatch cover elevated to a maximum height and theother hatch cover elevated sufficiently to clear its seal with the hatchcoaming.

FIG. 10 is a cross-sectional detail of the drive rack shown in FIG. 9.

FIG. 11 is an enlarged view at the centerline of the ship showing theadjacent hatch coamings and ventilating air duct.

Referring to the drawings, a typical vessel, incorporating the inventionof this application, is indicated by the numeral 5.

The vessel shown is of the double skin type, having relatively narrowside tanks 6 and a double bottom 7 as shown more clearly in FIG. 3.

As shown particularly in FIGS. land 2, the vessel 5 is provided withcargo holds 6, 7, 8, 9, l0 and 11. Each of the holds 7 through 10 are ofapproximately similar size and capacity, while holds 6 and 11 follow theform lines of the vessel fore and aft.

The open ship concept shown in the drawings has an arrangement of hatchopenings and hatch covers whereby the hatch openings and hatch coversare arranged in pairs transversely of the vessel with a minimum of spacebetween the pairs of hatch openings and covers, and between the hatchopenings and the sides of the vessel.

The hatch openings are indicated by the dotted lines 12 on FIG. 1 andthe hatch covers are indicated at 13.

v The hatch openings 12 are bounded above deck level by the coamings 14,and the coamings are supported by suitable brackets 15 for a purposewhich is more clearly illustrated in FIGS. 4 to 8.

The hatch openings 12, as shown more clearly in FIG. 3, are definedbelow the deck level by the outside longitudinal tanks or passagewaysl6, and amidships by the trunk 17.

The hatch covers 13 are of single piece pontoon construction, havingdeep peripheral edges 18, including an inverted channel 19 around theirlower peripheral edges into which a seal member 20 is inserted.

The top peripheral edge 21 of the coamings 14 is provided with anupwardly projecting peripheral compression bar member 22'adapted tosealingly engage with the seal member 20 in the hatch covers 13 whenthey are lowered into place over the hatch openings 12.

The hatch covers 13 are reinforced internally in order to maintain theirperipheral edges against distortion and to ensure that the seal providedby the mating members 20 and 22 will be uniformly gas tight around thewhole periphery of the covers and thereby assist in a proper controlledventilation of the holds of the vessel via the ventilating trunk 17.

Referring now to FIGS. 4, 5, 6, 9 and 10. In FIGS. 4, 5 and 6 it will beobserved that one hatch cover 13A of the two single piece, pontoon typehatch covers serving each of the holds of the vessel is elevated by fourhigh- Iift hydraulic jacks 23, while the other hatch cover 13B is liftedclear only sufficiently to break the gas-tight joint by four short-lifthydraulic jacks 24.

The hydraulic jacks 23 and 24 are located one pair on the forward facingside and one pair on the after facing side of each of the hatch covers13A and 138.

The high-lift hydraulic jacks 23 are mounted on brackets 25 on the sidesof the coamings 14A, the brackets 25 providing a housing into which theshafts 26 of the jacks are lowered in stowed position. The adjacent sidewalls of the hatch covers 13A are provided with blocks 27 with which theshafts 26 engage when the hatch covers are to be lifted.

The low-lift hydraulic jacks 24 are mounted on brackets 28 on the sidesof the coamings 14B and the shafts 29 of the jacks are provided with ashort length of lifting rail 30 for engagement with the rollers 31mounted for rotation on the adjacent side walls of the hatch covers 138.

Between the adjacent longitudinal coamings 13C and 13D there aremounted, for rotation, a pair of cog wheels 32, supported on thebrackets 33 spanning the space between the midship longitudinal coamings13C and 13D.

On each of the longitudinal sides 34 and 35 of the hatch covers 13B,there are pivotally mounted pairs of latch dogs 36 and 37. The latchdogs 36 engage with the teeth of the cog wheels 32 when the hatch cover138 is in the position shown in FIGS. 4 and 5, while the latch dogs 37engage with the cog wheels 32 when the hatch cover 138 has been moved tothe position shown in FIG. 6. When the hatch cover is in the closedposition, sealing the hatch opening, the latch dogs 36 and 37 can berotated to lie against the side walls 34 and 35 respectively of thehatch covers and be secured in that position by any suitable means so asnot to interfere with passageway on the deck alongside the hatches.

The cog wheels 32 are mounted on the shaft 38 which in turn is rotatedby a suitable drive, preferably by a hydraulic motor 29 as shown in FIG.9.

Secured to the lower edge of each of the transversely disposed sides 40of the hatch covers 133 is an inverted angle 41 and a series of equallyspaced pins 42 straddle the space between the member 43 of the angle 41and the adjacent surface of the wall 40 of the hatch cover. These pins42 are spaced apart so as to be engaged by the teeth of the cog wheels32 for movement of the hatch cover 13B horizontally when the shaft 38 isrotated. A series of pins 44, similar to and spaced at the same pitch asare the pins 42, are set in the latch dogs 36 and 37 In order to providefor easy transition of the hatch cover 133 from the position shown inFIG. 5 to the position shown in FIG. 6 and vice versa, there is providedrails 45 set on the upper edge of the transversely disposed walls 46 ofthe coamings 14A and 148. The rails 45 are aligned with the short lengthof rails 30 of the short-lift jacks 24 when these jacks are elevated todisengage the seal between the hatch cover 13B and the hatch coaming148.

By first engaging the latch dogs 36 with the cog wheels 32 and then, onrotation of the shaft 38 in an anti-clockwise direction, the hatch coveris moved transversely of the vessel from the position shown in FIG. 5 totake up the position shown in FIG. 6, with the latch dog 37 finallyengaging with the cog wheel 32 at the end of the movement of the hatchcover.

The jacks 23 under the hatch cover 13A can now be lowered to bring thathatch cover to rest on top of the hatch cover 138.

The half of the hold on the right hand side of the vessel, as shown inFIGS. 3, 5 and 6, can now be worked.

When it is desired to work the other half of the hold, i.e., the lefthand side as seen in FIGS. 4, 5 and 6, the two hatch covers 13A and 138can be moved together on the rails 45 in piggy back fashion,transversely of the vessel from left to right and remain supported onthe rails 45.

After both sides of the vessel have been worked and the hatch openingsare to be closed, the two hatch covers 13A and 13B are moved together tothe left into the position shown in FIG. 6. The jacks 23 are thenelevated to raise cover 13A clear of the hatch cover 13B. The hatchcover 138 is then moved to the right from under the hatch cover 13A tobring the wheels 31 over the short lengths of rail 30 and the jacks arelowered to bring the hatch cover into sealing engagement on the hatchcoaming.

The jacks 23 can now be lowered to bring the hatch covers 13A intosealing engagement on their hatch coaming. If should be noted that thejacks 23 and the blocks 27 on the hatch cover 13A are spaced outwardlyof the hatch cover sufficient to permit the hatch cover 13B to be movedfreely on the rails 45 and under the hatch cover 13A when the jacks 23are elevated as shown in FIG. 5.

The hydraulic jacks 23 and 24 and the hydraulic motors 39 can beindividually controlled from a remote position to enable the operationof lifting and transporting of the hatchcovers automatically and onlyinspection by a qualified person is required.

In order to give adequate support for the above described structure, itshould be noted that the peripheral walls of the coamings 14, as shownin FIGS. 3, 4, 5 and 6, are extended downwards below deck level. Theouter longitudinal walls of the coamings are extended downwards at 50 toform the inward facing walls of the longitudinal compartments 16 whichare a part of the double skin of the vessel and the inward longitudinalwalls of the coamings are extended downwards to form the longitudinalside walls 51 of the longitudinal ventilating trunk 17.

The transverse walls of the coamings 14 are extended downwards belowdeck level to form the transverse girders 52 extending between the outerlongitudinal compartments l6 and the centerline longitudinal ventilatingtrunk 17.

The strengthening of the peripheral area of the coamings 14 in themanner shown ensures that even under the most adverse conditions, thegas-tight seal between the hatch covers and the hatch coamings will bemaintained.

By ensuring a positive gas-tight seal of the hatch covers, a morepositive control of ventilation of the holds of the vessel can beeffected through the ventilating trunk 17.

Due to the relatively narrow space between the inwardly facinglongitudinal walls of the coamings, a short pipe connection can befitted between the ventilating trunk 17 andthe hold served by the twohatch openings 12.

Referring to FIG. 11, a short length of pipe 53, open at its lower endto the interior of the trunk 17, is topped by a reducer 54 and a crosstec 55, and branch connections are made to each of the hatch openings 12through the control valves 56 and the pipes 57.

The longitudinal ventilating trunk 17 extends lengthwise of the vessel,as shown in F IG. 1, to serve all of the holds 6 through 11 and is usedto convey air pumped into it by supply fans 58 located at the fore andaft ends of the vessel.

By this arrangement a common centerline longitudinal structure is usedto strengthen the vessel, to provide adequate support structure for thehatch covers and the means to move two adjacent hatch coverstransversely across the vessel, and to provide for short ventilating airconnections through the hold coamings at above deck level.

With the above described structure, including the "open ship hullstructure and hatch openings with the hatch cover sealing means and theease of movement of one or a pair of transversely adjacent hatch covers,the vessel can be rapidly converted to carry bulk cargo of either fluidor dry type and has the advantage of a double skin without the use ofupper-wing tanks whereby the holds can be used for maximum cubiccapacity. In addition, where fluid cargo is carried, the centerlineventilating trunk extends far enough down below deck level as to providefor an effective division of ullage below deck level to reduce theeffect of roll of the vessel on the fluid cargo.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. Hatch structure for twin hatch, open ship type vessels, the saidvessel having a main deck and twin hatch openings in the deck disposedtransversely of the vessel over a common hold, a hatch coaming abouteach of said twin hatch openings above deck level and longitudinal andtransverse reinforcing about each of said twin hatch openings below saiddeck, the longitudinal reinforcing structure under the midshiplongitudinal sides of the said coamings being common to the adjacentmidship sides of said hatch coamings, a pair of pontoon type hatchcovers, one for each of said twin hatch openings, high lift hydraulicjacks located adjacent two opposite transverse sides of a first of saidpair of hatch covers adapted to effect elevation of a first of saidhatch covers, low lift hydraulic jacks located adjacent two oppositetransverse sides of a second of said pair of hatch covers adapted toelevate said second hatch cover a distance sufficient to break the sealof the hatch cover with its coaming, a pair of rails disposedtransversely of the vessel and supported one on each opposite transversesides of the said coamings at the upper edge thereof, means to slidesaid second hatch cover transversely of the vessel to a positionunderneath an elevated said first hatch cover to uncover one of saidtwin hatch openings, and to slide the said pair of hatch coverstransversely of the vessel touncover the other of said twin hatchopenings.

2. Hatch structure as set forth in claim 1 in which the said secondhatch cover is fitted with two pairs of wheels for rolling engagementwith said pairs of rails when said second hatch cover is elevated bysaid low lift hydraulic jack.

3. Hatch structure as set forth in claim 2 in which the said low liftjacks include a short length of rail for engagement with the said wheelsduring elevation and lowering of the said second hatch cover.

4. Hatch structure as set forth in claim 3 in which the said shortlengths of rail is elevated into alignment with the said pair of railswhen the said low lift jacks are elevated.

5. Hatch structure as set forth in claim 1 in which the said pair ofrails are in sections whose end positions are spaced apart on eitherside of the vertical axis of the said low lift jacks and the said lowlift jacks include a section of rail, the said section of railsoccupying the space between said rails when the said low lift jacks areelevated.

6. Hatch structure as set forth in claim 1 in which the said means toslide said second hatch cover includes a pair of latch dogs pivotallymounted on each of the longitudinally disposed sides of the said hatchcover, a series of equally spaced pins mounted on the transverselydisposed sides of the said hatch cover and on said latch dogs, and apair of cog wheels are mounted for rotation between the adjacentlongitudinally disposed sides of said first and second hatch covers whenthe said latch dogs are pivoted into engagement with said cog wheels.

7. Hatch structure as set forth in claim 6 in which the said cog wheelsare mounted on a shaft disposed longitudinally of the vessel, the saidshaft being rotatable in brackets bridging the space between thecoamings of said twin hatch openings, and a drive motor is connected tothe said shaft.

8. Hatch structure as set forth in claim 1 in which the seal between thesaid hatch covers and the said coamings is gas tight and includes anelastomeric seal member seated in a peripheral channel around thelowermost peripheral edge of the hatch cover, and a peripheral memberprojecting upwards from the uppermost peripheral edge of the saidcoamings.

9. Hatch structure as set forth in claim 6 in which the said low lifthydraulic jacks are elevated a distance sufficient to lift the saidsecond of said pair of hatch covers clear of the peripheral memberprojecting upwards from the coaming.

l0. Hatch structure as set forth in claim 1 in which the saidlongitudinal reinforcing structure at midships of the vessel is aventilating trunk extending lengthwise of the vessel, and a ventilatingconnection is located between each adjacent pair of hatch coamings, thesaid ventilating connection being open to said trunk and through saidcoamings to the interior of the hold at said hatch openings.

11. Hatch structure as set forth in claim 10 in which the saidventilating connection includes a pair of shutoff valves, the saidshut-offvalves individually closing the connections from said trunk tothe hold at said hatch openings.

12. Hatch structure for twin hatch, open ship type vessels, the saidvessel having a main deck and twin hatch openings in the deck disposedtransversely of the vessel over a common hold, a hatch coaming abouteach of said twin hatch openings above deck level and longitudinal andtransverse reinforcing about each of said twin hatch openings below saiddeck, the longitudinal reinforcing structure under the midshiplongitudinal sides of the said coamings being common to the adjacentmidship sides of said hatch coamings, a pair of pontoon type hatchcovers, one for each of said twin hatch openings, means to form a sealbetween the said hatch covers and the peripheral edge of thecorresponding hatch coamings, high lift hydraulic jacks located adjacenttwo opposite transverse sides of a first of said pair of hatch coversadapted to effect elevation of a first of said hatch covers, low lifthydraulic jacks located adjacent two opposite transverse sides of asecond of said pair of hatch covers adapted to elevate said second hatchcover a distance sufficient to break the seal of the hatch cover withits coaming, a pair of rails disposed transversely of the vessel andsupported one on each opposite transverse sides of the said coamings atthe upper edge thereof, means to slide said second hatch covertransversely of the vessel to a position underneath an elevated saidfirst hatch cover to uncover one of said twin hatch openings, and toslide the said pair of hatch covers transversely of the vessel touncover the other of said twin hatch openings.

1. Hatch structure for twin hatch, open ship type vessels, the saidvessel having a main deck and twin hatch openings in the deck disposedtransversely of the vessel over a common hold, a hatch coaming abouteach of said twin hatch openings above deck level and longitudinal andtransverse reinforcing about each of said twin hatch openings below saiddeck, the longitudinal reinforcing structure under the midshiplongitudinal sides of the said coamings being common to the adjacentmidship sides of said hatch coamings, a pair of pontoon type hatchcovers, one for each of said twin hatch openings, high lift hydraulicjacks located adjacent two opposite transverse sides of a first of saidpair of hatch covers adapted to effect elevation of a first of saidhatch covers, low lift hydraulic jacks located adjacent two oppositetransverse sides of a second of said pair of hatch covers adapted toelevate said second hatch cover a distance sufficient to break the sealof the hatch cover with its coaming, a pair of rails disposedtransversely of the vessel and supported one on each opposite transversesides of the said coamings at the upper edge thereof, means to slidesaid second hatch cover transversely of the vessel to a positionunderneath an elevated said first hatch cover to uncover one of saidtwin hatch openings, and to slide the said pair of hatch coverstransversely of the vessel to uncover the other of said twin hatchopenings.
 2. Hatch structure as set forth in claim 1 in which the saidsecond hatch cover is fitted with two pairs of wheels for rollingengagement with said pairs of rails when said second hatch cover iselevated by said low lift hydraulic jack.
 3. Hatch structure as setforth in claim 2 in which the said low lift jacks include a short lengthof rail for engagement with the said wheels during elevation andlowering of the said second hatch cover.
 4. Hatch structure as set forthin claim 3 in which the said short lengths of rail is elevated intoalignment with the said pair of rails when the said low lift jacks areelevated.
 5. Hatch structure as set forth in claim 1 in which the saidpair of rails are in sections whose end positions are spaced apart oneither side of the vertical axis of the said low lift jacks and the saidlow lift jacks include A section of rail, the said section of railsoccupying the space between said rails when the said low lift jacks areelevated.
 6. Hatch structure as set forth in claim 1 in which the saidmeans to slide said second hatch cover includes a pair of latch dogspivotally mounted on each of the longitudinally disposed sides of thesaid hatch cover, a series of equally spaced pins mounted on thetransversely disposed sides of the said hatch cover and on said latchdogs, and a pair of cog wheels are mounted for rotation between theadjacent longitudinally disposed sides of said first and second hatchcovers when the said latch dogs are pivoted into engagement with saidcog wheels.
 7. Hatch structure as set forth in claim 6 in which the saidcog wheels are mounted on a shaft disposed longitudinally of the vessel,the said shaft being rotatable in brackets bridging the space betweenthe coamings of said twin hatch openings, and a drive motor is connectedto the said shaft.
 8. Hatch structure as set forth in claim 1 in whichthe seal between the said hatch covers and the said coamings is gastight and includes an elastomeric seal member seated in a peripheralchannel around the lowermost peripheral edge of the hatch cover, and aperipheral member projecting upwards from the uppermost peripheral edgeof the said coamings.
 9. Hatch structure as set forth in claim 6 inwhich the said low lift hydraulic jacks are elevated a distancesufficient to lift the said second of said pair of hatch covers clear ofthe peripheral member projecting upwards from the coaming.
 10. Hatchstructure as set forth in claim 1 in which the said longitudinalreinforcing structure at midships of the vessel is a ventilating trunkextending lengthwise of the vessel, and a ventilating connection islocated between each adjacent pair of hatch coamings, the saidventilating connection being open to said trunk and through saidcoamings to the interior of the hold at said hatch openings.
 11. Hatchstructure as set forth in claim 10 in which the said ventilatingconnection includes a pair of shut-off valves, the said shut-off valvesindividually closing the connections from said trunk to the hold at saidhatch openings.
 12. Hatch structure for twin hatch, open ship typevessels, the said vessel having a main deck and twin hatch openings inthe deck disposed transversely of the vessel over a common hold, a hatchcoaming about each of said twin hatch openings above deck level andlongitudinal and transverse reinforcing about each of said twin hatchopenings below said deck, the longitudinal reinforcing structure underthe midship longitudinal sides of the said coamings being common to theadjacent midship sides of said hatch coamings, a pair of pontoon typehatch covers, one for each of said twin hatch openings, means to form aseal between the said hatch covers and the peripheral edge of thecorresponding hatch coamings, high lift hydraulic jacks located adjacenttwo opposite transverse sides of a first of said pair of hatch coversadapted to effect elevation of a first of said hatch covers, low lifthydraulic jacks located adjacent two opposite transverse sides of asecond of said pair of hatch covers adapted to elevate said second hatchcover a distance sufficient to break the seal of the hatch cover withits coaming, a pair of rails disposed transversely of the vessel andsupported one on each opposite transverse sides of the said coamings atthe upper edge thereof, means to slide said second hatch covertransversely of the vessel to a position underneath an elevated saidfirst hatch cover to uncover one of said twin hatch openings, and toslide the said pair of hatch covers transversely of the vessel touncover the other of said twin hatch openings.